Good question, Jo Ann! The answer is that you cannot buy it in the US nor anywhere else. Golden rice is not being developed for profit. It's not intended to be a consumer product to be sold in supermarkets. Instead, it's part of a humanitarian project and will be given away for free to farmers in countries where vitamin A deficiency is a problem.
As to why farmers in these countries don't have it yet, it's due to politics and a lot of regulatory red tape. But countries like Bangladesh and the Philippines are making progress and hopefully at least there it will be made available to everyone soon.